The widespread use of switched mode power supplies and other factors has contributed to unwanted harmonics placed on the power system to which such loads are connected. Harmonic line current standards such as IEC 555-2 for switched mode power supplies has resulted in the adoption of designs by several manufacturers of power supplies which achieve unity power factor. Various designs are available for improving the input waveform quality of power converters, including the use of an active switching converter and/or the use of passive filter elements at the power input stage. See e.g., K. Kit Sum, "Power Factor Correction for Single Phase Input Power Supplies" PCIM, December 1989, pp. 18-23, and J. J. Spangler, "A Power Factor Corrected MOSFET Multiple Output Flyback Switching Power Supply," Proceedings of Power Conversion International PCI Conference, October 1985.
A widely utilized circuit structure includes a boost converter between the high voltage rectifier at the utility end and the DC bus of the DC to DC converter. An example of such a system is shown in U.S. Pat. No. 4,437,146 to Carpenter entitled "Boost Power Supply Having Power Factor Correction Circuit". In such systems, the DC bus capacitor provides intermediate energy storage to smooth out the input power fluctuations. The DC to DC converter, in turn, provides high frequency isolation, voltage step down, and line and load regulation of the output voltage. The two transistor forward converter has been commonly used for the DC to DC converter at medium to high power levels. The use of two independent power stages inherently decouples the problems of input current shaping and output regulation, allowing these to be treated independently, and each is addressed by a different power stage. While such approaches are effective, the additional switching components required add cost to the overall system.